Man-made fibers are often supplied as tows, which are bundles of generally parallel continuous filaments, each such bundle containing a large number of such filaments, generally well over 500, e.g. 6,000 to 500,000. In the manufacture of tows, sub-tows may be formed by joining together a plurality of continuous filaments. A number of sub-tows, each containing only a fraction of the filaments desired in the main tow, are usually fed side-by-side to a draw frame where they are stretched, in a manner well known in the art, to develop the desired physical properties (e.g. high tenacity and stiffness). The sub-tows are then combined and the resulting tow is fed to a crimping device, which is preferably of the stuffer box type, where the filaments are crimped, and then, in an untensioned condition, onto a belt which transfers the crimped tow continuously through an oven maintained at a temperature sufficiently high to permanently set the crimp, but not high enough to damage or melt the filaments.
The draw frame, crimping device and oven are utilized most efficiently when the tow passing therethrough contains a very large number of filaments, e.g. two whose total denier is over about 50,000 and which contains over about 10,000 filaments. However, for certain end uses the crimped, heat set tow may be lower or higher in weight or filament count. For example, in the production of cigarette filters from tow, the number of filaments may be as low as 5,000 and the total denier as low as 35,000.
Cigarette filters may be formed from such crimped continuous filament tows. Upon being received by the filter manufacturer, the tow is opened and the crimps of the filaments deregistered, utilizing a process such as that disclosed in U.S. Pat. No. 3,156,016 to Dunlap et al. The tow is fed along a predetermined path and is subjected to a differential gripping action between a plurality of points spaced from one another both longitudinally and transversely of the path, so that certain laterally spaced sections of the tow are positively gripped relative to other laterally spaced sections of the tow, alternating with the gripped sections, which are not gripped at all or are gripped at different relative points. In this manner there is produced, as a function of the differential positive gripping of the tow, a relative shifting of adjacent filaments longitudinally of the tow, whereby the crimps are moved out of registry with one another.
The deregistered tow is then fed through a chamber in which a plasticizer is applied to the tow. The tow is thereafter treated to reduce its cross-sectional area until it is approximately equal to the cross-sectional area of a cigarette. The condensed mass is formed into a coherent structure, e.g. by wrapping paper around it and/or by curing, and is ultimately cut into suitable plug lengths for incorporation into cigarettes.
It has been found that the filters produced are not all identical as far as filtering action is concerned. The filters differ somewhat in weight, in filtering efficiency and in their resistance to gas flow therethrough. After smoking, some filters show a degree of uneven darkening, which indicates a somewhat non-uniform passage of smoke therethrough. The more darkened areas identify zones through which the smoke has been preferentially drawn. Tests have shown that a significant cause of these non-uniformities in the cigarette filters result from variations in the total denier and number of ends in the tow. This variation results, to a degree, from undetected broken ends in the tow forming process. Clearly, uniform cigarette filters cannot be produced from a tow which is not substantially uniform.
Therefore, in the manufacture of continuous filament tow, filament breakage is a serious problem. If filament breakage does occur during the production of the tow, the product will have gaps or discontinuities within it which will adversely affect the uniformity of products made from the tow, unless steps are taken to compensate for the broken ends and thereby maintain a uniform number of ends.
Deregistered, opened tows may be utilized for purposes other than cigarette filters. Such tows are especially suitable for the production of nonwoven fibrous sheet-like structures such as may be used as sanitary napkin cover fabrics, pillow stuffing material, filling for sleeping bags or mattress covers, and the like. These products are superior to conventional nonwovens in their freedom from loose fiber ends, in their uniformity (e.g. freedom from thick or thin spots) and strength in the longitudinal direction. Clearly, in order to produce a uniform product, a substantially uniform tow is required.
The continuous feeding of filamentary material is, therefore, critical to the production of substantially uniform tow. Suitable means must be provided for detecting individual breaks in the filaments making up the tow. In addition, the delivery of a continuous strand of filamentary material must be assured. To accomplish this, it is essential that appropriate means be utilized to ensure the feeding of a new filamentary strand or strands to a given tow line upon the breakage of one or more strands of continuous filamentary material.
Certain types of feeding mechanisms are presently available to retain the leading end of a standby supply of filamentary material in readiness to be fed to an appropriate feeding mechanism upon the breakage or exhaustion of one or more strands of filamentary material. One such mechanism is disclosed in U.S. Pat. No. 3,128,026 which utilizes electrical means to effect certain shifting arrangements which are utilized to transfer the driving effect of a feeding mechanism from a first strand of filamentary material to a second strand. Another such mechanism is illustrated in U.S. Pat. No. 3,519,181. A mechanism is provided for feeding continuous strandular material, utilizing guide means defining a path and means for forwarding the strandular material along this path. A standby supply of strandular material is provided with a supporting means engaging and positioning its leading end. A control, sensitive to the exhaustion of the first strand, because of the loss of contact therewith, operates to initiate movement of the support means for the first strand to introduce the second strand into engagement with the feed means.
Both of these apparatus require an arrangement for transferring the driving effect of a feed mechanism from the exhausted or broken strandular material to the standby end for insertion into the multifilament strandular material being produced. Of necessity, this results in a certain lag time before the standby end is actually inserted. Under such circumstances, it is clear that a gap can occur in the supplying of an end to the multifilament product, thereby providing for non-uniformity of the final product.
Accordingly, to produce substantially uniform continuous filament tows, it is necessary to provide means for ensuring the continuation of a continuous filamentary material from additional supplies upon the breakage of one or more of the original strands. In order to ensure such a continuous supply of strandular material, the supply means must be immediately warned of the breaking of the strandular material and set into action means for quickly forwarding a new supply of filamentary material to replace the disappearing end of the broken strand.